Google’s Vic Gundotra just announced that, about a year after the service went live, 400,000 people have now activated their Google+ accounts. More importantly, though, Gundotra also notes that Google+ now has more than 100 million monthly active users. What’s interesting about this statement is that this number specifically refers to Google+ and not, like the numbers Google previously announced, to the number of active Google+ users who also use other Google products.

Until now, Google always shied away from providing detailed statistics about Google+’s active user base and instead preferred to talk about how the service had become Google’s “social spine.” Google also often talked about how Google+ now had more than 170 million users and how 60 percent of those signed in daily, but those were pure vanity metrics and didn’t actually reflect Google+’s real user base. Instead they referred to how often users with Google+ accounts signed in to other Google services.

Google was widely criticized for these numbers but, until now, refused to release any detailed numbers for Google+ as a destination site. It’s still worth noting that we don’t know how Google defines “monthly active users,” but at least these numbers refer to the actual social network and not just the “social spine.”

(Update: We talked to Google and the definition is pretty straightforward. It’s, as the name implies, users who visit the site or use the mobile app at least once per month. This means Google isn’t playing any games here and counts offsite +1s or similar actions as active use.)

Here is Gundotra’s full post:

This week we also hit an important milestone–over 400,000,000 people have upgraded to Google+. It was only a year ago that we opened public sign-up, and we couldn’t have imagined that so many people would join in just 12 months. While Google+ is all about creating a better experience across Google, it’s also a destination. And here too, I’m happy to report that we have just crossed 100,000,000 monthly active users on Google+ (plus.google.com and mobile app).